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What does "3-sigma" type of event mean? For example,

Hurricane Sandy is a 3-sigma type of event.

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closed as general reference by Roaring Fish, Andrew Leach, Mark Beadles, kiamlaluno, Zairja Oct 30 '12 at 19:15

This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information.If this question can be reworded to fit the rules in the help center, please edit the question.

    
So, probability 0.003, or 1 in 300. Figuratively, very rare. You will have to read more to see what they really mean. Maybe a storm like this will hit New York City about once in 300 years. –  GEdgar Oct 30 '12 at 15:22
    

2 Answers 2

up vote 5 down vote accepted

The greek letter sigma is used as a measure (the standard deviation) of how much some series of events vary around the average. Events much bigger or smaller than the average are rarer than events close to the average

For a certain type of statistical distribution you can say that 1/3 of the events will be more than 1 sigma from the average, 1/22 more than 2, 1/370 more than 3 etc etc. It's only valid for things which follow this particular curve - which hurricanes probably don't - so in this case it's a bit of scientific sounding jargon to sound good in a news cast.

It's popular in the US because there was a business philosophy called 6sigma based on the idea that your business should be 6 standard deviations better than average ( ie 1/500,000,000)

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1  
Good answer, but I think most scientists believe that most natural phenomena follow a normal distribution, at least the trends of death and destruction from hurricanes do, so I don't think the caveat that "hurricanes probably don't" is needed. –  ghoppe Oct 30 '12 at 16:26
    
Hurricane strength probably isn't Gaussian because there are hard limits at the upper and lower end of the available thermal power. Damage might be Gaussian because it sums a very large number of essentially random events –  mgb Dec 29 '12 at 6:32

Uncle Google has the answer.

Three Sigma Systems

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Why is it Uncle? Why not little brother? –  Tim Oct 30 '12 at 15:11
    
Clever retort, but you should summarize the content of the link in case it rots in the future. –  ghoppe Oct 30 '12 at 16:16
    
@ghoppe ~ there are plenty of spare links. No need to worry! I didn't summarise because it not really a language question, which is why it is now closed. I was in two minds whether to answer at all, but then decided it would be more civil to provide the information even if it is only a link. –  Roaring Fish Oct 31 '12 at 0:57

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